Bcl-xL is an anti-apoptotic member of the Bcl-2 family that is abundantly expressed in adult neurons and is overexpressed in a number of human cancers, where Bcl-xL contributes importantly to tumor cell survival. Anti-apoptotic Bcl-2 proteins are thought to inhibit apoptosis by directly or indirectly inhibiting pro-apoptotic Bcl-2 family proteins Bax and Bak. If unleashed, Bax and Bak compromise the integrity of the outer mitochondrial membrane, releasing cytochrome c into the cytoplasm to activate the caspases responsible for apoptotic cell death. However, a growing body of evidence suggests that this model of Bcl-2 family function is incomplete, and that Bcl-2 proteins have important functions in healthy cells, particularly in neurons. Furthermore, these non-canonical functions of Bcl-2 proteins appear to be critical for mitochondrial function and neuronal activity, as well as cell viability. Therefore, contrary to previous opinion that cell death regulators act late in the course of disease, long after neuronal function is impaired, Bcl-xL-mediated functions are likely to be critical for functions that are lost in the earliest phases of neurological dysfunction. However, the functions of Bcl-2 family proteins are only rarely studied in healthy cells. We propose to study the effects of Bcl-xL on mitochondrial functions in neurons using biochemical approaches and microscopy to challenge the current dogma regarding Bcl-2 protein localization and to identify the mitochondrial components relevant to Bcl-xL function (Aim 1). We will also investigate the mechanisms underlying the effects of Bcl-xL or mitochondrial dynamics (fission, fusion and biomass control) and energetics (Aim 2), and will determine the role of Bcl-xL in membrane fusion using cellular and in vitro models (Aim 3).